The present invention relates to methods and apparatus for extracting an information signal from an encoded audio signal.
There are various motivations to permanently or indelibly incorporate information signals into audio signals, referred to as “watermarking.” Such an audio watermark may provide, for example, an indication of authorship, content, lineage, existence of copyright, or the like for the audio signals so marked. Alternatively, other information may be incorporated into audio signals either concerning the signal itself or unrelated to it. The information may be incorporated in an audio signal for various purposes, such as identification or as an address or command, whether or not related to the signal itself.
There is considerable interest in encoding audio signals with information to produce encoded audio signals having substantially the same perceptible characteristics as the original unencoded audio signals. Recent successful techniques exploit the psychoacoustic masking effect of the human auditory system whereby certain sounds are humanly imperceptible when received along with other sounds.
One particularly successful utilization of the psychoacoustic masking effect is described in U.S. Pat. No. 5,450,490 and U.S. Pat. No. 5,764,763 (Jensen et al.) in which information is represented by a multiple-frequency code signal which is incorporated into an audio signal based upon the masking ability of the audio signal. The encoded audio signal is suitable for broadcast transmission and reception as well as for recording and reproduction. When received the audio signal is then processed to detect the presence of the multiple-frequency code signal. Sometimes, only a portion of the multiple-frequency code signal, e.g., a number of single frequency code components, inserted into the original audio signal are detected in the received audio signal. If a sufficient quantity of code components is detected, the information signal itself may be recovered.
Generally, an acoustic signal having low amplitude levels will have only minimal capacity, if any at all, to acoustically mask an information signal. For example, such low amplitude levels can occur during a pause in a conversation, during an interlude between segments of music, or even within certain types of music. During a lengthy period of low amplitude levels, it may be difficult to incorporate a code signal in an audio signal without causing the encoded audio signal to differ from the original in an acoustically perceptible manner.
A further problem is the occurrence of burst errors during the transmission or reproduction of encoded audio signals. Burst errors may appear as temporally contiguous segments of signal error. Such errors generally are unpredictable and substantially affect the content of an encoded audio signal. Burst errors typically arise from failure in a transmission channel or reproduction device due to severe external interferences, such as an overlapping of signals from different transmission channels, an occurrence of system power spikes, an interruption in normal operations, an introduction of noise contamination (intentionally or otherwise), and the like. In a transmission system, such circumstances may cause a portion of the transmitted encoded audio signals to be entirely unreceivable or significantly altered. Absent retransmission of the encoded audio signal, the affected portion of the encoded audio may be wholly unrecoverable, while in other instances alterations to the encoded audio signal may render the embedded information signal undetectable. In many applications, such as radio and television broadcasting, real-time retransmission of encoded audio signals is simply unfeasible.
In systems for acoustically reproducing audio signals recorded on media, a variety of factors may cause burst errors in the reproduced acoustic signal. Commonly, an irregularity in the recording media, caused by damage, obstruction, or wear, results in certain portions of recorded audio signals being unreproducable or significantly altered upon reproduction. Also, misalignment of or interference with the recording or reproducing mechanism relative to the recording medium can cause burst-type errors during an acoustic reproduction of recorded audio signals. Further, the acoustic limitations of a speaker as well as the acoustic characteristics of the listening environment may result in spatial irregularities in the distribution of acoustic energy. Such irregularities may cause burst errors to occur in received acoustic signals, interfering with code recovery.